services: configuration: Add user-defined sanitizer support.

This changes the 'custom-serializer' field into a generic
'extra-args' field that can be extended to support new literals.
Within extra-args, the literals 'sanitizer' and 'serializer' allow
for user-defined sanitization and serialization procedures respectively.
The 'empty-serializer' was also added as a literal to be used as before.

To prevent confusion between the new “explicit” style of specifying
a sanitizer, and the old “implicit” style, the latter has been
deprecated, and a warning is issued if it is encountered.

* gnu/services/configuration.scm (define-configuration-helper):
Rename 'custom-serializer' to 'extra-args'.  Add support for literals
'sanitizer', 'serializer' and 'empty-serializer'.  Rename procedure
'field-sanitizer' to 'default-field-sanitizer' to avoid syntax clash.
Only define default field sanitizers if user-defined ones are absent.
(normalize-extra-args): New variable.
(<configuration-field>)[sanitizer]: New field.
* doc/guix.texi (Complex Configurations): Document the newly added
literals.
* tests/services/configuration.scm: Add tests for the new literals.

Signed-off-by: Liliana Marie Prikler <liliana.prikler@gmail.com>

1 files changed, 27 insertions, 2 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index a58ea8f9ec..495a930d0d 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -41219,7 +41219,7 @@ A clause can have one of the following forms:
 (@var{field-name}
  (@var{type} @var{default-value})
  @var{documentation}
- @var{serializer})
+ (serializer @var{serializer}))
 
 (@var{field-name}
  (@var{type})
@@ -41228,7 +41228,18 @@ A clause can have one of the following forms:
 (@var{field-name}
  (@var{type})
  @var{documentation}
- @var{serializer})
+ (serializer @var{serializer}))
+
+(@var{field-name}
+ (@var{type})
+ @var{documentation}
+ (sanitizer @var{sanitizer})
+
+(@var{field-name}
+ (@var{type})
+ @var{documentation}
+ (sanitizer @var{sanitizer})
+ (serializer @var{serializer}))
 @end example
 
 @var{field-name} is an identifier that denotes the name of the field in
@@ -41251,6 +41262,20 @@ an object of the record type.
 @var{documentation} is a string formatted with Texinfo syntax which
 should provide a description of what setting this field does.
 
+@var{sanitizer} is a procedure which takes one argument,
+a user-supplied value, and returns a ``sanitized'' value for the field.
+If no sanitizer is specified, a default sanitizer is used, which raises
+an error if the value is not of type @var{type}.
+
+An example of a sanitizer for a field that accepts both strings and
+symbols looks like this:
+@lisp
+(define (sanitize-foo value)
+  (cond ((string? value) value)
+        ((symbol? value) (symbol->string value))
+        (else (error "bad value"))))
+@end lisp
+
 @var{serializer} is the name of a procedure which takes two arguments,
 the first is the name of the field, and the second is the value
 corresponding to the field.  The procedure should return a string or